In recent years, the liquid crystal display elements are expected to be useful not only in the conventional application to personal computer monitors but also in application to ordinary color televisions. The color reproduction range of the color liquid crystal display elements is determined by colors of light emitted from the red, green and blue pixels and, where chromaticity points of the respective color pixels in the CIE XYZ calorimetric system are represented by (xR, yR), (xG, yG) and (xB, yB), the color reproduction range is represented by an area of a triangle defined by these three points on an x-y chromaticity diagram. Namely, the larger the area of this triangle, the more vivid color image the display elements reproduce. The area of this triangle is normally expressed by a ratio of the area of the triangle to an area of a reference triangle formed by three points of the three primary colors, red (0.67, 0.33), green (0.21, 0.71) and blue (0.14, 0.08), in the standard system defined by U.S. National Television System Committee (NTSC) (in unit of %, which will be referred to hereinafter as “NTSC ratio”). The ordinary notebook computers have the values of approximately 40 to 50%, the desktop computer monitors the values of 50 to 60%, and the existing liquid crystal TVs the values of approximately 70%.
In the color liquid crystal display elements, the color filter extracts only wavelengths in necessary regions from the emission distribution of the backlight, to provide the red, green and blue pixels.
Methods for production of this color filter proposed heretofore include such methods as dyeing, pigment dispersion, electrodeposition, printing, ink jetting and so on. The colorants for coloring used to be dyes, but are now pigments in terms of reliability and durability as liquid crystal display elements. Accordingly, at present, the pigment dispersion is most commonly used as a method for production of the color filter from the viewpoint of productivity and performance. In general, when the same colorant is used, the NTSC ratio and the brightness are in a trade-off relation, and the colorant is suitably selected for use depending on the particular application. Namely, if it is attempted to increase the NTSC ratio by adjusting the color filter in order to reproduce a vivid color image, the screen tends to be dark. Inversely, if the brightness is increased, the NTSC ratio tends to be low, and it tends to be difficult to reproduce a vivid image.
On the other hand, as a backlight, it has been common to employ one using as a light source a cold-cathode tube with emission wavelengths in the red, green and blue wavelength regions and using a light guide plate for converting light emitted from this cold-cathode tube, into white area light source. In recent years, a light emitting diode (LED) has been used, since it has a longer operating life, requires no inverter, presents high brightness, is mercury free, etc.
Here, in a conventional LED type backlight, a blue emission from LED and a yellow phosphor obtained by excitation by means of such a blue emission were used as a white area light source.
However, in the above light source, the phosphor was yellow, whereby emission with wavelength unnecessary from the viewpoint of the color purity of red and green was substantial, and it was difficult to obtain a display with high color reproducibility (High Gamut). Here, it is at least in principle possible to increase the color purity of red and green by cutting off light with unnecessary wavelength by means of a color filter, but as mentioned above, if it is attempted to increase the NTSC ratio by adjusting the color filter in order to reproduce the vivid color image, the majority of emission of the backlight will be cut off, whereby there has been a problem that the brightness decreases substantially. Especially, by this method, emission of red decreases substantially, whereby it has been practically impossible to reproduce a strongly reddish color.
In order to overcome this problem, a method of combining red-, green- and blue-emitting LEDs (Non-Patent Document 1) has been proposed, and by this method, a display having an extremely high color reproducibility has been prepared on a trial basis. However, in such a color image display device, LED chips independent for red, green and blue, respectively, are combined, whereby is there have been problems such that 1) it takes time and labor to mount them, 2) since the respective LED chips for red, green and blue are disposed at finite distances, it is required to take the distance of a light guide plate to be long to sufficiently mix emitted lights from the respective LED chips, and 3) since the white chromaticity is adjusted by combining the integral multiple of the respective LED chips, adjustment of the white balance can not be continuously carried out.
Further, a color image display device having an NTSC ratio of at least 60% has been disclosed which is constituted by a combination of a blue or deep blue LED and a phosphor (Patent Document 2). With this color image display device, a high color reproducibility may be attained as compared with the above mentioned yellow phosphor, but emitted lights with wavelengths which are unnecessary from the viewpoint of the color purity of red and green, are substantial, and a still higher color reproducibility has been desired.
Non-Patent Document 1: Monthly display, April 2003 issue (p 42-46)
Patent Document 2: WO2005/111707